Human temperatures are not always a steady 98.6°F. Some people run warmer than others, and children are generally warmer than adults. Circadian rhythms, the weather, and even what your kids are wearing may affect a temperature reading. So if your child looks sick, is acting weird, or your thermometer is reading something higher than 100°F, it’s best to check in with your doctor.
There are many different types of thermometers, and perhaps more surprisingly, there’s fascinating physics behind these little engineering marvels. Alok and Bethany run through five of your temperature-taking options—from rectal to forehead. They show how to best use thermometers, posit what kind and which method may be ideal for your child (depending on age and the ability to sit still!), and explain the science of how thermometers work.
Your typical digital thermometer uses an electronic heat sensor to measure body temperature, Alok explains. A digital tympanic thermometer is “exhilarating” and “fascinating,” he says, and because it must accurately measure the temperature inside of an ear canal, it’s also called an ear thermometer. An alcohol thermometer is also an option, but make sure you have one designed for humans (your child is not a freezer). And lastly, there are the vintage relics of the past: mercury thermometers.
Alok and Bethany’s message? Don't take these fever-monitoring devices for granted. Not only are they crucial tools to help you or a health care provider monitor your child's wellbeing and check for fever; they're also fascinating tools.
Next time you show off your physics of thermometers knowledge, we bet you’ll get a standing ovation.
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How to Take Your Kid's Temperature
Published: October 13, 2020
Alok Patel: Rectal temperature? Let’s do it, Mom!
Bethany Van Delft: They’re usually like, “Nooooo. Nooooooo.”
Alok Patel: Did you know, Bethany, that the most accurate way to check your baby’s temperature is with a rectal thermometer, which goes in the rectum?
Bethany Van Delft: Yes, I did know. But do you have any tips—like, the best way to take a temperature rectally? Because, maybe this is exclusive to my children—none of my babies were like, “Sure, put that up there. I’ll cooperate!”
Alok Patel: They weren’t like, “Rectal temperature? Let’s do it, Mom!”
Bethany Van Delft: They’re usually like, “Nooooo. Nooooo.”
Alok Patel: Just maniac in place. So what’s baby’s name?
Bethany Van Delft: This is Mia. Mia Baby.
Alok Patel: Mia Baby, Mia Baby. Now with one hand, spread the buttocks. You know where it goes. You want to cover that silver tip at the end of it.
Bethany Van Delft: Ok.
Alok Patel: This is what we usually consider to be the most accurate way to take a temperature, especially in young kids under the age of 3 months where a fever could potentially be serious.
Bethany Van Delft: Beep.
Alok Patel: Beep. Baby doesn’t have a fever ‘cause she’s made of cloth. In young, young, young babies, we strongly advise using a rectal. Let’s say you don’t have one.
Bethany Van Delft: If your baby doesn’t have a rectum, then you can use...
Alok Patel: If your baby doesn’t have a rectum, we’ve got some other problems to discuss. So next, you said your babies sometimes don’t like to get rectal temperatures. You can do an armpit temperature or an axillary temperature. There you go! Now make sure you have a good seal. You’re going to get a beep on this one also. Now one thing to take note of is that an axillary or armpit temperature is going to run slightly cooler than the rectal temperature.
And that same thermometer, after you clean it, can also be used as an oral temperature as well. This is obviously harder to do in younger babies who aren’t going to cooperate. Do your kids cooperate with oral temperatures?
Bethany Van Delft: With nothing. Nothing on Earth.
Alok Patel: But in the kids that do, make sure your kid bites down. I’m sorry, does not bite down! But closes his or her mouth down on the thermometer.
Bethany Van Delft: When I was little, thermometers were glass with mercury in them.
Alok Patel: Were you little in the ’30s?
Bethany Van Delft: I was little in 1919.
Alok Patel: Bethany! She probably doesn’t have a fever because we’ve established the fact that she’s not human. Mia, you’re human, shh. But let’s talk about the second most accurate way to check a temperature, and that’s with a temporal thermometer.
So ditch that old school oral one. Temporal. You’re gonna go right in the middle of Mia’s forehead, she doesn’t care, push the scan button. You’re gonna move along the middle of the forehead towards the top of the ear, stop right at the hairline.
Bethany Van Delft: That’s like, the easiest. I can’t just stick with this all the time?
Alok Patel: A lot of parents do like it. Now let’s say Mia is now 6 months old. You can start checking her ear.
Bethany Van Delft: Can we do your ear temperature?
Alok Patel: How do your kids do with ear temps?
Bethany Van Delft: They love this, they like pushing the buttons, and doing it themselves, they love this.
Alok Patel: They actually love it?
Bethany Van Delft: They love it. They’re super weird.
Alok Patel: Playtime is “take my temperature in my ear, Mom.”
Bethany Van Delft: “Take my temperature in my ear!”
Alok Patel: Now you’re a pro at this. You want to make sure you have a good seal so you’re using the proper ear thermometer. What does it say?
Bethany Van Delft: No fever cause she’s still a doll.
Alok Patel: There’s a lot of variation. Children are warmer than adults, some people run warmer than others, there’s circadian rhythms, there’s the weather outside, there’s what your kids are wearing. What I would again worry about is if your kid looks sick, is acting weird or your thermometer is reading something higher than that 100 range, just check in with your doctor.
Bethany Van Delft: Some people just hot, right?
Alok Patel: Some people just hot.
Bethany Van Delft: Some people just hot.
Alok Patel: Maybe not fever hot, but like, warm.
Bethany Van Delft: Alright.
Alok Patel: There's so many different types of thermometers, and I know you're all really fascinated about the physics behind these little engineering marvels. I hear you. So let's run through them, what they can be used for and how they work.
First up, the regular digital thermometer. Now, this uses an electronic heat sensor to basically measure body temperature. It can be used in the mouth, armpits and you guessed it, the rectum. There's a few pros. It's fast and it's easy. It's pretty accurate. However, if you're trying to get an oral reading, it's not as accurate if somebody has to breathe through their mouth. You need to get that good seal. They also are battery operated and they can die, kind of like this one. You also have to absolutely make sure you're keeping them clean for obvious reasons when you think about where they go. You want to make sure you’re using at least a 60% alcohol-based solution. You can use a dilution of bleach also. Make sure you’re not only cleaning and disinfecting the tip, but also the entire body of the thermometer.
Next up is the exhilarating, fascinating digital ear thermometer to accurately measure temperature inside of an ear canal, which is why it’s also called a tympanic thermometer. Here's how it works. Every object gives off a little bit of infrared radiation. The infrared radiation coming off the ear canal goes back to this thermometer, it hits something called the thermopile. That infrared radiation then gets transferred into heat, the heat then goes to an electronic sensor, and you get a temperature reading. Now there are a few pros with this. It’s quick and it's comfortable. It's also really cool too. But a few cons. It can’t be used in really young children, in their tiny little ear canals. And earwax can also interfere with you trying to get an accurate measurement. Also, as always, you do want to make sure you're keeping this thing clean. And what you want to do is just use a little bit of alcohol or a disinfectant solution with a cotton ball or a Q-tip and clean that tip.
But your child is three months old and you want to use this really cool infrared technology? Ha, you're in luck because now you've a temporal artery thermometer. This is like the forehead thermometer. A lot of people have seen it uses infrared technology to basically take thousands of readings from the temporal artery. And it uses all those readings to give you an accurate measurement. Obviously, this is fast, it's accurate and it's really easy to use. And you can use it on children as young as three months. One of the biggest cons, however, is it’s pretty expensive. And as always, make sure you’re keeping the thermometer clean. You don’t want to be using this on multiple people and spreading germs, from one infrared-obsessed child to another.
Next up, we have the alcohol thermometer. You have to make sure you're using an alcohol thermometer designed for humans. This one is made for a freezer. Your child is not a freezer. Now, alcohol thermometers basically work because alcohol expands when it gets really hot. You can actually get a measurement based on the expansion. Now, one downside to it, which may not affect you, is that it doesn't give you an accurate reading past 172F. If any human out there has lived with a temperature of 172F, you're not a human. I hope it’s never an issue for you. Last but not least, I'm really excited to say this phrase, please clean your alcohol thermometer with an alcohol-based solution.
Then there's mercury thermometers, which are a relic of the past, and now they're like cool vintage medical tools. So a mercury thermometer works similar to an alcohol thermometer. Basically, when the mercury is warm or hot, it expands, it rises on the tube, and it gives you a temperature measurement. Now, you should not be using this anymore. This, I should not even be holding and playing with because they can break, the mercury can spill out, and it's toxic. So in the end, don't take thermometers for granted. Not only are they crucial tools to help you or a health care provider monitor your child's health, check for fever, they're also fascinating engineering marvels. And now any time you go out, you go to the bar, you can talk to people about how much you know about the physics of these little fever-monitoring devices. And watch yourself get a standing ovation.
But let’s say your child is really young and you have a tympanic thermometer, but you still want to get an accurate reading using this very cool infrared technology. Voila. Why aren’t you going on? Alright, cool. We’ll, you’re in luck. What the—?
Hosted by: Alok Patel and Bethany Van Delft
Producer/Director: Ari Daniel
Producer/Camera: Emily Zendt
Production Assistance: Diego Arenas, Glorie Martinez, Christina Monnen, Arlo Pérez, Drew Powell, Emma Uk, Madeline Weir
Senior Digital Editor: Sukee Bennett
Rights Manager: Hannah Gotwals
Business Manager: Elisabeth Frele
Managing Producer: Kristine Allington
Coordinating Producer: Elizabeth Benjes
Director of Audience Development: Dante Graves
Director of Public Relations: Jennifer Welsh
Legal and Business Affairs: Susan Rosen and Eric Brass
Director, Business Operations and Finance: Laurie Cahalane
Executive Producers: Julia Cort and Chris Schmidt
Fatma Dedeoglu, MD
Jonathan S. Hausmann, MD
BruceBlaus / Wikimedia / CC BY 3.0
iStock.com / Rafael Abdrakhmanov
NASA Goddard Space Flight Center
Cigaro30 / freesound / CC BY 3.0
Special Thanks: David Condon